Return structure when accessing a singletPropertyValue

Return structure when accessing a singlet PropertyValue from the PropertySource SPI. So

String get/String key)

will be changed to

PropertyValue get(String key);

This allows the SPI to deal with additional metadata in a similar way as it is possible, when accessing all properties using Map<String,String> getProperties();.
A PropertyValue hereby is designed as an immutable value object containing

• the requested key (String)
• the value (String)
• the Map<String,String> representing any kinf of additional context data.